1. Field of the Invention
The present invention relates to a wireless communication system using multiple transmission antennas, and more particularly to differential space-time block codes (D-STBCs) with a high symbol transmission rate.
2. Description of the Related Art
Generally, the primary concern in communications is how efficiently and reliably data can be transmitted through a channel. Recent developments for next generation multimedia mobile communication systems require high-speed communication systems capable of processing and transmitting various types of information such as image data and wireless data, in addition to voice communication To do so, it is necessary to enhance system efficiency by employing a suitable channel coding scheme for the system.
As opposed to a wired channel environment, a radio channel environment is prone to errors from various causes, such as multi-path interference, shadowing, propagation attenuation, time-varying noises, and interference and fading. These errors cause information loss, making the communication system less reliable. As a result, research and study in this area focuses on a solution to the above-described problems.
Tarokh suggested space-time block codes (STBCs) as a part of the above-described studies, that provide the maximum order of a diversity and facilitate maximum likelihood (ML) decoding with only simple linear processing in a reception terminal in a wireless communication system employing multiple transmit antennas.
In the STBCs, symbols input as blocks are output through a transmission matrix. Herein, the columns and rows of the transmission matrix represent time and antennas, respectively. Therefore, symbols on the columns of the transmission matrix represent signals transmitted from different antennas at the same time, and symbols on the rows of the transmission matrix represent signals transmitted from one antenna at different times. Also, there are merits such as maximum diversity and simple decoding in the STBCs. That is, the STBCs can obtain a large signal to noise ratio (SNR) gain without increasing system complexity, as compared with a system using only one antenna.
As a result, methods where a transmission terminal acquires transmit diversity gain by using the STBCs to overcome fading and improve reliability of transmission signals, have been actively researched and studied.
The conventional STBCs described above are used when the reception terminal has channel information, and real systems estimate channel information with a training symbol. However, a transmission method excluding use of channel information may be more advantageous when it is difficult to estimate a channel or where channel estimation creates excessive overhead due to rapid change of the channel.
Tarokh suggested differential space-time block codes (D-STBCs) with respect to two transmit antennas. Also, the suggestion for the D-STBCs of Tarokh shows a case of employing any number of transmit antennas. Generally, such D-STBCs have performance degradation of about 3 dB and similar encoding/decoding complexity as compared with the conventional coherent space-time block codes.
However, in transmission of a complex signal, the conventional D-STBC has a transmission rate of 1 symbol/transmission with respect to two transmit antennas and has a transmission rate of 0.75 symbol/transmission with respect to at least three transmit antennas. The above described restriction in the transmission rate occurs because a transmit antenna is utilized to acquire a diversity gain only. Accordingly, it is more advantageous in regards of system performance that a transmission terminal pursues a (spatial) multiplexing gain while sacrificing the diversity gain to obtain a high transmission rate by transmitting a greater number of symbols where the reception terminal employs multiple receive antennas.
Hereinafter, description about the conventional D-STBC will be given with reference to FIG. 1.
FIG. 1 is a schematic block diagram for the conventional D-STBC. Specifically, FIG. 1 is a block diagram showing a structure of the transmission terminal employing the conventional D-STBC.
Referring to FIG. 1, the transmission terminal includes a symbol mapper 101, a space-time encoder 103, a multiplier 105, a delayer 107, and a distributor 108. Also, FIG. 1 illustrates a structure of the transmission terminal of a D-STBC system having N transmit antennas.
Regarding operation of the transmission terminal, input data (s1, s2, . . . , sk) provided through the symbol mapper 101 are created as a signal Sv for v-th transmission block having P×N symbols by the space-time encoder 103. The transmission matrix Dv+1 is obtained by multiplying the signal Sv by the delayed version of transmission matrix Dv for v-th block which is provided by the delay block 107. The encoded symbol in the transmission matrix Dv+1 is transmitted through each antenna, that is, a first antenna to an Nth antenna. In addition, the N antennas simultaneously transmit N symbols for every time slot with the same symbol interval during P time slots.
As described above, the transmission terminal of the system employing the conventional D-STBC includes two processes. The first is a process of encoding data to be transmitted by using STBCs and the second, a process of differentially encoding and transmitting a space-time block coded matrix.
Hereinafter, it is assumed that a wireless communication system employs eight or fewer transmission, or transmit antennas. Also, for the purpose of explanation, it is assumed the reception terminal employs one reception, or receive antenna. However, it is possible that the reception terminal be capable of employing multiple receive antennas.
Under the above assumptions, when two transmit antennas are used, two symbol intervals are transmitted as one unit, and when three or four transmit antennas are used, four symbol intervals are transmitted as one unit. Further, when five to eight transmit antennas are used, eight symbol intervals are transmitted as one unit. Herein, matrices such as Equation (1) to Equation (4) are defined as:
                                          G            22                    =                      [                                                                                a                    1                                                                                        a                    2                                                                                                                    -                                          a                      2                                                                                                            a                    1                                                                        ]                          ,                              B            22                    =                      [                                                                                a                    1                                                                                        a                    2                                                                                                                    -                                          a                      2                      *                                                                                                            a                    1                    *                                                                        ]                                              (        1        )                                                      G            43                    =                      [                                                                                a                    1                                                                                        a                    2                                                                                        a                    3                                                                                                                    -                                          a                      2                                                                                                            a                    1                                                                                        -                                          a                      4                                                                                                                                        -                                          a                      3                                                                                                            a                    4                                                                                        a                    1                                                                                                                    -                                          a                      4                                                                                                            -                                          a                      3                                                                                                            a                    2                                                                        ]                          ,                              G            44                    =                      [                                                                                a                    1                                                                                        a                    2                                                                                        a                    3                                                                                        a                    4                                                                                                                    -                                          a                      2                                                                                                            a                    1                                                                                        -                                          a                      4                                                                                                            a                    3                                                                                                                    -                                          a                      3                                                                                                            a                    4                                                                                        a                    1                                                                                        -                                          a                      2                                                                                                                                        -                                          a                      4                                                                                                            -                                          a                      3                                                                                                            a                    2                                                                                        a                    1                                                                        ]                                              (        2        )                                                      B            43                    =                      [                                                                                a                    1                                                                                        a                    2                                                                                        a                    3                                                                                                                    -                                          a                      2                      *                                                                                                            a                    1                    *                                                                    0                                                                                                  -                                          a                      3                      *                                                                                        0                                                                      a                    1                    *                                                                                                0                                                                      -                                          a                      3                      *                                                                                                            a                    2                    *                                                                        ]                          ,                              B            44                    =                      [                                                                                a                    1                                                                                        a                    2                                                                                        a                    3                                                                    0                                                                                                  -                                          a                      2                      *                                                                                                            a                    1                    *                                                                    0                                                                      a                    3                                                                                                                    -                                          a                      3                      *                                                                                        0                                                                      a                    1                    *                                                                                        -                                          a                      2                                                                                                                    0                                                                      -                                          a                      3                      *                                                                                                            a                    2                    *                                                                                        a                    1                                                                        ]                                              (        3        )                                          G          88                =                  [                                                                      a                  1                                                                              a                  2                                                                              a                  3                                                                              a                  4                                                                              a                  5                                                                              a                  6                                                                              a                  7                                                                              a                  8                                                                                                      -                                      a                    2                                                                                                a                  1                                                                              a                  4                                                                              -                                      a                    3                                                                                                a                  6                                                                              -                                      a                    5                                                                                                -                                      a                    8                                                                                                a                  7                                                                                                      -                                      a                    3                                                                                                -                                      a                    4                                                                                                a                  1                                                                              a                  2                                                                              a                  7                                                                              a                  8                                                                              -                                      a                    5                                                                                                -                                      a                    6                                                                                                                        -                                      a                    4                                                                                                a                  3                                                                              -                                      a                    2                                                                                                a                  1                                                                              a                  8                                                                              -                                      a                    7                                                                                                a                  6                                                                              -                                      a                    5                                                                                                                        -                                      a                    5                                                                                                -                                      a                    6                                                                                                -                                      a                    7                                                                                                -                                      a                    8                                                                                                a                  1                                                                              a                  2                                                                              a                  3                                                                              a                  4                                                                                                      -                                      a                    6                                                                                                a                  5                                                                              -                                      a                    8                                                                                                a                  7                                                                              -                                      a                    2                                                                                                a                  1                                                                              -                                      a                    4                                                                                                a                  3                                                                                                      -                                      a                    7                                                                                                a                  8                                                                              a                  5                                                                              -                                      a                    6                                                                                                -                                      a                    3                                                                                                a                  4                                                                              a                  1                                                                              -                                      a                    2                                                                                                                        -                                      a                    8                                                                                                -                                      a                    7                                                                                                a                  6                                                                              a                  5                                                                              -                                      a                    4                                                                                                -                                      a                    3                                                                                                a                  2                                                                              a                  1                                                              ]                                    (        4        )            
Herein, the matrix G43 is obtained by using the first three columns of the matrix G44. The matrix B43 is obtained by using the first three columns of the matrix B44. A transmission matrix G8N for a predetermined constant N (N=5, 6, 7, 8) is obtained as a sub-matrix formed by using the first N columns of the matrix G88.
The matrices B22, B43, and B44 create D-STBCs for transmitting complex signals through N number of transmit antennas, where N=2, 3, and 4, respectively. In addition, the matrices G22, G43, G44 and G8N create D-STBCs for transmitting real signals through N number of transmit antennas, where N=2, 3, 4, 5, 6, 7, and 8, respectively. Herein, the matrices are used for coherent STBCs, designed in such a manner that the coherent STBCs have orthogonality.
Used signal constellation previously determines a transmission matrix to be selected depending on the number of transmit antennas used in the transmission terminal and depending on whether a complex or real signal is transmitted. Hereinafter, an example of using the matrix B43 from among the matrices, that is, transmitting the complex signal through three antennas, will be described. The manner described below may be applied to a case of transmitting the real signal, or using a different number of transmit antennas.
First, signal transmission starts from selection of a predetermined symbol S1(S1=(s1,1, s1,2, s1,3)) and substitution of the selected symbol for the transmission matrix B431. An (i.t)th element of the matrix B431 in a first block is transmitted through an ith transmit antenna at a tth time. At this time, symbols transmitted in the first block have no information and are not known to the reception terminal.
Hereinafter, reflexive description about symbols transmitted after a vth block will be described.
That is, it is assumed that the transmission matrix B43v in the vth block was transmitted in the same manner as the first block. In addition, inputted binary data are modulated to a v+1th symbols, that is, symbols Sv+1=(sv+1,1, sv+1,2, sv+1,3), and the modulated symbols are substituted for elements ‘a1’, ‘a2’, and ‘a3’ of the matrix B44 so as to create a matrix S44v+1 in a v+1th block. Then, the matrix B43v, having been transmitted in a previous block, is multiplied by the matrix S44v+1 so as to create a new matrix B43v+1 to be transmitted in the (v+1)th block to perform a differential encoding function.
In a summary of this, the matrix B43v+1 can be defined by Equation (5):B43v+1=S44v+1B43v  (5)
Herein, the (i,t)th element of the matrix B43v+1 is transmitted through an ith transmit antenna at a tth time. The reason for the substitution of elements ‘a1’, ‘a2’, and ‘a3’ of the matrix B44 is that the matrix B44 has the same number of rows as the matrix B43 and is the smallest matrix having orthogonality. Herein, if a differential space-time block coding is realized by using the matrix G8N for each transmit antenna when the number of the transmit antennas is N (N=5, 6, 7, or 8), the matrix S44v+1 in Equation (5) must be replaced with the matrix S88v+1 in which eight symbols Sv+1(Sv+1=(sv+1,1, sv+1,2, . . . , sv+1,8)) are substituted for elements of the matrix G88.
Below, the case where a signal transmitted from the transmission terminal is received in the reception terminal as described above will be explained.
First, if a channel gain h(h=(h1, h2, h3)T) is time-invariant with respect to two connected blocks, a received signal in a vth block is defined as Equation (6):Xv=B43vh+Wv  (6)
Herein, the Xv(Xv=(xv,1, xv,2, xv,3, xv,4)T) denotes a signal received during four symbol intervals, and the Wv(Wv=(wv,1, wv,2, wv,3, wv,4)T) denotes a noise signal. Similarly, the signal Xv+1 is received in the (v+1)th block as defined in Equation (7):Xv+1=B43v+1h+Wv+1  (7)
Herein, when Equation (5) and Equation (6) are substituted into Equation (7), the receive signal Xv+1 in the (v+1)th block is represented as Equation (8):Xv+1=B43v+1h+Wv+1=S44v+1B43vh+Wv+1=S44v+1Xv+Nv+1  (8)
Herein, the noise component Nv+1 can be defined as Equation (9):Nv+1=−S44v+1Wv+Wv+1  (9)
Herein, since the matrix S44v+1 has orthogonality, the variance of the noise component in Equation (8), that is, an additive white Gaussian noise (AWGN) is double and the distribution of the noise component is steadily maintained.
The receive signal in Equation (8) has the same form as a receive signal of the conventional coherent STBC using the matrix B44. That is, if the matrix Xv is a channel gain, Equation (8) has the same form as mathematics formulas of the conventional reception terminal receiving the receive signal Xv+1. Accordingly, since the variance of the noise signal is double, the above described method can acquire diversity gains proportional to the number of transmit antennas and simplify the reception terminal structure even though performance is degraded by about 3 dB as compared with the coherent STBC.
However, the conventional D-STBC peculiarly arranges symbols in a transmission matrix for maintaining orthogonality of STBCs, so that the maximum transmission rate is limited. In addition, when the above described method uses three or four transmission antennas with respect to a complex signal by way of example, the maximum transmission rate is 3/4(0.75 symbol/transmission) because three symbols are transmitted during four symbol intervals. Therefore, the transmission rate is lowered. Furthermore, although the conventional technique suggests a QAM transmission scheme replacing a PSK transmission scheme, the symbol transmission rate is still 0.75 symbol/transmission.
Accordingly, a new D-STB is desirable, that has a symbol transmission rate higher than that of the conventional D-STBC while maintaining orthogonality of the conventional D-STBC.